Say a fella wants to build an SET monoblock based on the 300B..

The Audio Note amplifier kit is granted more expensive than the other good 300b kits mentioned above. In my opinion it’s money well spent.
1 Very sturdy and high quality aluminum chassis.
2 Very good quality standard parts. High quality power supply.
3 The option to upgrade capacitors and resistors and especially the output transformers (C-cores are available)
4Tube rectified circuit
5 Proven track record x 25 years.
6 The potential to build an upper tier high end quality 300b SET.
7 300b offers more flexibility in matching speakers due to higher power compared to the 45 and 2A3 tube options.
Charles

I agree with Charles. I have had 2 of their aamps built by their sanctioned builder, Pete Fulton, whose build quality is immaculate. I also had him build one of their tube DACs. In each case I was very satisfied and they were totally reliable. 
You Might be able to do it yourself, but for th additional 10% charge, you have the peace of mind of knowing it was done right, and the resale value will be better as well. 

I didn't know the additional charge to build the Audio Note kit amplifier is only 10%. That's seems to me a very reasonable deal unless someone wants the satisfaction/challenge of doing the assembly themselves. 
Charles 

It is reasonable Charles, and if you look at their site, you'll see the beautiful job that Pete does. Besides that, the average guy doesn't have testing equipment. Also, if you want to use certain capacitors or resistors etc, they will accommodate that.

Also critical when planning for a low power amplifier is to know in advance what speakers you will want to drive with them.  It goes without saying that speaker efficiency and speaker impedance are of prime importance.